A stent is commonly used to expand a constricted portion of a vessel, to maintain an open passageway through a vessel, or to maintain the integrity of other structures of the body such as are found in the respiratory or biliary tracts. For instance, a stent may be placed in an artery to prevent a reclosing of the artery at the treated point following a procedure such as balloon angioplasty or other procedures designed to reduce or remove arterial plaque. Such a stent must have a diameter sufficient to expand or maintain open the passageway and to remain firmly in position where placed without danger of displacement by the fluid flowing through it or by the movement of the wall of the passageway. To introduce a suitably sized stent using conventional angiographic catheters, the general procedure is to compress the stent to a size smaller than finally desired, to transfer the compressed stent to the desired location in the body, and finally to expand the stent to its uncompressed state.
A coil-shaped stent is a self-supporting structure, offering the most structural support for the least amount of structure material. Each separate coil tends to support its neighboring coil or coils. Although the coil shape is, therefore, highly desirable, current methods of compressing the coil to allow delivery to the desired body site do not provide sufficient control over the shape of the stent during its expansion to the uncompressed state.
Various means for compressing and delivering a helically coiled stent to the desired location have been developed Maass et al., U.S. Pat. No. 4,553,545 discloses a mechanical method of reducing the diameter of the coils by tightly winding the coils to a smaller diameter than the final desired diameter. The tightly wound stent is inserted into the body and mechanically unwound for placement. This method requires complicated delivery apparatus and cannot be easily used where the stent is to be placed in a position which must necessarily be reached by a twisting or turning pathway. In addition, a guide wire often is placed in the artery during the plaque removal procedure to assist in placing the stent at the proper location, and a guide wire cannot readily be used with this specialized delivery system. Thus, placement of the stent at the precise, desired location is difficult. Furthermore, the tightly wound coils do not always unwind evenly and adjacent coils can become intermeshed, which prevents the stent from becoming fully unwound.
Another method of expanding a compressed helically coiled stent depends on the prior formation of the stent from a shape memory alloy material. A coil of a shape memory alloy, such as nitinol, can be formed by wrapping the alloy wire around a mandrel to form a stent of the final desired length and diameter. This stent is then heated and held at the elevated temperature for the time required to train the wire into the desired shape. The cooled stent can then be compressed, and upon exposure to its transition temperature, will resume its trained shape. The composition of the alloy can be adjusted to achieve a transition temperature that is either higher or lower than normal body temperature. A compressed stent having a transition temperature in the range of 115-125.degree. F. can be placed in the body and be returned to its uncompressed state with a warm saline solution. Conversely, a stent having a transition temperature lower than normal body temperature can be kept in its compressed state by bathing with a cool saline solution until the stent is placed at its desired location after which it is allowed to expand as it warms to body temperature.
Dotter, U.S. Pat. No. 4,503,569 discloses a helically wound stent made of the shape memory alloy nitinol. The stent is either tightly wound or elongated to reduce its diameter, inserted in the body to the desired position, then heated to its transition temperature to cause it to return it to its trained-in diameter. However, controlled unwinding and reforming of every individual coil to the final desired shape can be very difficult. The coils of the stent may not recover in controlled fashion. In addition, delivery of a tightly wound stent through a turning body passageway is difficult.